Paper Rater is an excellent resource for students and teachers. It is 100% free and works like this:
1) You paste your essay/prac/letter/email/job application/whatever in their search box
2) Fill in some boxes, year level, type of text, your reference list
3) Click "Get Report"
and you get a full report that highlights any problems (in a very engaging interface!)
It even can help you increase you vocabulary by sending you to Vocab Builder.
Their blurb "PaperRater.com is a free resource, developed and maintained by linguistics professionals and graduate students. PaperRater.com is used by schools and universities in over 46 countries to help students improve their writing.
PaperRater.com combines the power of natural language processing (NLP), artificial intelligence (AI), machine learning, information retrieval (IR), computational linguistics, data mining, and advanced pattern matching (APM). We offer the most powerful writing tool available on the internet today."
I tested this blog post and it gave me a B, however on the plus side it noted:
"Excellent work! Your usage of sophisticated words is on par with other well-written papers! Nevertheless, you may still wish to use our Vocab Builder to maintain your edge."
Test it and tell your students!